This invention relates to automatic adaptive equalizers such as are used in high speed quadrature-amplitude modulation (QAM) data communications equipment. (The term QAM is used herein broadly to include all systems in which the transmitted signal can be represented as the superposition of two modulated signals, each being obtained by pulse-amplitude modulation of a signal sequence on a sinusoidal carrier, the two signal sequences being generated in synchronism at the same rate, and the two carriers being of the same frequency but 90.degree. apart in phase. QAM thus includes a wide variety of double sideband systems, including pure phase modulation and combined amplitude and phase modulation.) The invention is particularly useful in a multi-point data transmission system in which a central station communicates with a number of remote stations.
It is common practice to adapt the equalizer in a receiver to the particular channel and transmitter from which it is to receive a signal burst, by using a known training sequence as part of a preamble of the burst. One review of prior equalization art is Proakis and Miller, IEEE Trans. Inf. Theo., Vol. IT-15, No. 4, 1969.
Typically, the preamble also includes signals for initial adjustment of other receiver parameters. In particular, the first segment of the preamble typically contains a sequence for initialization of the timing recovery circuitry, to optimize the timing epoch .tau., where output signals are put out by the receiver at times kT + .tau., k = 0, 1, . . . , the receiver being adapted to receive signals as 1/T signals per second. The timing recovery sequence precedes the equalizer training sequence in conventional systems because improper setting of .tau. may interfere with proper equalization.